anime didn't do this, you can keep watching
lack of training/using it caused your reasoning neurocircuitry to shrink (also quit drugs and get checked for depression, they can cause this too)
you probably weren't that smart to begin with but you can still improve by making sure your life is healthy enough (sleep,diet,exercise,etc.) and doing some maths / chess / puzzles / abstract reading / whatever makes you train both memory and ability to compute mentally.